Prisma has attracted several of the real heavyweights in European aerospace. One of these is the French national space board CNES, Centre National d'Études Spatiales. For them Prisma is an opportunity to test new technologies, which would be difficult for them to do themselves.
 
The French national space board CNES has taken a very active part in the Prisma-project. For CNES, technology showcases, such as Prisma, are difficult to manage within their own organisation.
 
”There is no science involved, it is only about technology and CNES doesn’t work in that way. It is the science that is the major driving force behind potential projects within CNES”, says Michel Delpech, who is responsible for the CNES formation-flight experiment on Prisma.
 
CNES can, however, participate as a collaborative partner. That is why Prisma fills a gap for CNES which otherwise would be difficult to fill. Michel Delpech was one of the international participants who, together with a number of Swedes, gathered in the Swedish Space Corporation’s premises in Solna on the 21st of November for a demonstration of Prisma and of the project’s progress. The occasion also gave the Swedish and international press the opportunity to have a look and ask questions.

New perspectives
Prisma will for the first time test advanced formation-flight in space. Why then is formation-flight such a big thing?

“It is the only way to achieve the high resolutions necessary for us to be able to see, for example, planets similar to the Earth around other stars”, says Michel Delpech. “Formation-flight definitely gives rise to new perspectives”.
 
It is true. Instrumentation on Earth will always be limited by the cloudy atmosphere, the magnetosphere and other features our planet has and which make it a good and pleasant place to live. One can build larger telescopes, which is happening (even in that area technology is improving) and one can refine detection instrumentation.
 
“But sooner or later one will come to the end of the road”, says Michel Delpech.
 
There are also plans to use interferometers in the opposite direction, to observe the Earth. One can, for example, measure earth movements with much greater accuracy and measure how the tectonic plates on the Earth’s surface move against each other, how volcanoes "sigh" and settle between eruptions and even measure ground subsidence and evaluate collapse risks.

Extreme accuracy
For this one has to be above the atmosphere; and launching a telescope or other detectors that are enormous into space is difficult, if not nigh on impossible. The interferometer principle is one way of getting round this. In an interferometer one links together several instruments and in this way a vastly improved resolution is achieved. The drawback is that the instruments must be extremely accurately positioned in relation to each other. The satellites must be aware of their positions relative to each other to within less than a millimetre. This accuracy in positioning is beyond that which can be achieved by control from the ground. The satellites must therefore be able to solve this themselves; they must be able to perform autonomous formation-flight. 
 
Prisma gives CNES an opportunity of testing instruments that are intended to be used within other projects for formation-flight. Prisma also presents an opportunity for testing the systems for steering and navigational control, as well as other onboard processes. 
 
“One shouldn’t however forget the ground aspects”, states Michel Delpech. “Even there reside new concepts and approaches, which are important to gain experience of and obtain proof that they work”. 
 
Psychologically important
Michel Delpech is of the opinion that one shouldn’t underestimate the psychological side of a project like Prisma.
 
“After such a project one can say "Now we know that we can handle this!". It is no longer just hopes and dreams”.
 
Frederic Teston, ESA’s project leader for the organisation’s own test-satellite for formation-flight, Proba-3, agrees: 
 
“Prisma can help in breaking though a psychological barrier”.
 
Frederic Teston points out that formation-flight is really something completely novel and that one can therefore expect some unanticipated surprises. A technology showcase such as Prisma can therefore be extremely valuable in advance of the hugely challenging future projects such as Darwin, Xeus and Simbol-X.

 Kim Bergström, November 2007